(1) Field of the Invention
The present invention relates to a speech signal processing system and, more particularly, to an echo suppressor for attenuating or eliminating an echo arising from a speech signal on a receiving line shifting round partly to a transmitting line and thus returning to a receiver which is a transmitting source of the speech signal in a two- to four-wire conversion circuit, for example, in a telephone communication system.
(2) Description of the Prior Art
Many a proposal has already been made on an art for decreasing or eliminating an echo caused by an impedance mismatching in a two- to four-wire conversion circuit (hybrid circuit) in the telephone communication system. In a typical echo suppressing art known already hitherto, an arrangement is such that an attenuator is inserted halfway of a receiving line, an echo signal coming back to a receiver of a near end user is attenuated by actuating the attenuator while the near end user is in talk, a talk commencement of a far end user is detected to remove the attenuator, thereby monitoring a speech signal level on both receiving line and transmitting line. There is known a method wherein a switch for cutting off an echo path is provided on the transmitting line, to control the switch according to the level of incoming signals The echo suppressing system of this kind is described, for example, in Japanese Patent Laid-Open No. 80029/1984 (U.S. Ser. No. 437,093) and Japanese Patent Laid-Open No. 124131/1985.
In the field of a digitized telephone communication system, a low bit rate speech transmission system wherein a redundancy information is removed when encoding the speech signal is known. A speech compression is attained by analyzing an input speech at every fixed period (frame cycle) of, for example, 10 to 20 msec., obtaining parameters indicating spectrum information and excitation information characterizing the speech, and transmitting these on a predetermined format. In this case, the excitation information is classified into amplitude information indicating a loudness of the speech, pitch information indicating an intonation of the speech, and information indicating a coarseness of the speech and the like, all of which are parametrized accordingly.
In such low bit rate speech transmission system, a relatively large transmission delay at, for example, several 10 msec. or so is involved for encoding and decoding the speech. Consequently, from employing a system wherein speech signal levels of the receiving line and the transmitting line are compared with each other, and an attenuator is inserted in the receiving line during the period of time when a near end user is in talk, an echo signal corresponding to a tail portion of the speech uttered by the user reaches a receiver as kept alive, and thus a speech quality is deteriorated.